This invention relates to non-aqueous coating and ink formulations for use on flexible film or paper packages for food, which require aseptic packaging conditions. Printed and coated packaging materials, and methods of aseptic packaging, as well as the aseptic packages are also disclosed.
Foods are generally packaged in portion sizes, which may be single serving portions or multiple serving portions. Disposable packaging is used extensively in all areas of the food industry. The ability to print information directly on such packages offers cost savings, but the packaging processes to which the materials must be subjected are demanding. Typically foods such as milk and juice require packaging under aseptic or sterile conditions, which means that any coatings and/or printing inks used to label such packaging must also endure such conditions. The main product packaging materials comprise flexible films, such as polyethylene alone or in combination with other polymer resins. However, paper-based laminated packaging materials usually comprising paper, polyethylene and aluminum foil are also employed.
Examples of paper-based laminated packaging materials are well known in the art, and include, for example the materials disclosed in U.S. Pat. No. 5,635,011 issued Jun. 3, 1997 to Rosen; U.S. Pat. No. 5,531,060 issued Jul. 2, 1996 to Fayet et al; U.S. Pat. No. 4,495,016 issued Jan. 28, 1985 to Viberg et al; U.S. Pat. No. 4,461,667 issued Jul. 24, 1984 to Pupp; and U.S. Pat. No. 4,424,260 issued Jan. 3, 1994 to Pupp.
Examples of flexible film packaging, heat-sealable polymeric films are typically fabricated into disposable packages for containing flowable materials including for example liquids such as milk, fruit juices and the like. The preferred form of package is a pillow-shaped pouch that is often manufactured on a vertical form, fill and seal apparatus with the flowable material being placed in the pouch as part of its manufacture. It is often the practice to place multiples of such pouches into larger bags upon which product information may be printed. In such instance, the larger bag is not in direct contact with the food that is packaged and as a result the ink requirements are substantially different from those where the pouch itself contains printed matter about product contents and the printed film is in contact with the food.
Thus, where a food product is to be packaged and sterilized, the film used to make the pouch or package for such product must also be sterilized. In addition, the printed film must be capable of not only withstanding the aseptic packaging conditions required for sterilized food products, but also must be capable of withstanding the rigours of manufacture, transport, storage and handling from packaging to the point of sale. These requirements also apply to products that are not sterilized but are subjected to aseptic packaging conditions.
The requirements for an ink for applying to the surface of polymer films are very much dependent on the manufacturing conditions and the product being packaged. The printed film must endure any converting operations, such as printing, laminating, coating, and slitting into rolls of correct width. Previously, in order to produce printed packages that would be capable of sterilization in, for example, a solution of hot hydrogen peroxide, an extra layer of film was applied via a lamination step to cover and protect the surface of the printed film.
The inks and coatings for use with aseptic packaging materials, in particular film pouches must meet a number of different requirements and hence the development of suitable formulations has posed a challenge. For example the peroxide used to sterilize printed film can cause the ink to debond from the polymer surface, dissolve, fade or bleed so that the graphic image on the pouch deteriorates unacceptably. Components in the ink may cause the peroxide sterilant to decompose at an accelerated rate that will shorten its useable life and may pose a serious safety hazard if the containing vessel is not well vented.
Most ink systems, whether solvent-based or water-based, are not suited for printed films which are sterilized in hot peroxide. A number of criteria for identifying suitable ink systems are as follows. The printed films must have the following properties: scuff-resistance; bleed-resistance and colourfastness in the presence of hot hydrogen peroxide solutions; safe for packaging liquid foods like milk, juice and water over a lifetime of ingestion of the fluid product; and non-reactive with hot hydrogen peroxide solution nor catalyze its decomposition reaction.
A wide variety of flexible packaging printing inks and coatings has been proposed in the patent literature and are available commercially. Typically, these inks are printed by rotary letter press printing using flexible rubber plates or by gravure printing using engraved chrome plated cylinders on a wide variety of substrates including plastic films selected from polyolefins, polyesters, polystyrene, cellophane, cellulose acetate and the like. See, for example, U.S. Pat. No. 5,338,785 issued Aug. 16, 1994 to Catena, et al.; U.S. Pat. No. 5,658,968 issued Aug. 19, 1997 to Catena, et al.; and U.S. Pat. No. 4,321,185 to Benitez issued Mar. 23, 1982.
Coating and ink formulations for printing on paper-based laminated aseptic packaging materials are also known. This type of packaging material is sold commercially by Tetra Pak International AB and by Combibloc Inc. An example of a package made from such material is a carton that may be a reclosable aseptic carton. Normally the carton is pre-sterilized and milk after being subjected to an ultra-high temperature (UHT) process is placed in the carton and then hermetically sealed to prevent any contamination.
Typically the laminated packages or cartons are composed of three materials: paper (70%), polyethylene (24%) and aluminum foil (6%). The paper provides stiffness, strength and the block shape. The polyethylene used on the innermost layer seals the package. The aluminum foil provides light and oxygen barrier. A protective polyethylene coating on the exterior keeps the carton dry and covers the printed surface. This coating may be laminated or extrusion coated. The printing of the carton surface may be by roto-flexo printing which requires a laminating and cutting step after the printing.
Examples of paper-based laminated packaging materials can be found in the patent literature. These materials are used to produce aseptic packages. Examples of typical materials are disclosed in U.S. Pat. No. 4,424,260 issued Jan. 3, 1984 to Pupp; U.S. Pat. No. 4,416,667 issued Jul. 24, 1984 to Pupp and U.S. Pat. No. 4,495,016 issued Jan. 22, 1985 to Viberg et al. The last patent references a printed layer but the structure of the packaging material and the process does not involve exposing the printed layer to the sterilizing conditions used in the packaging operation. The process is rendered more complicated because this must be avoided.
An important aspect of the ink and coating formulations of the present invention is that while polyamide resins, and co-solvent type polyamide resins have previously been incorporated as base varnishes in ink and coating systems, no one has previously recognized the necessity for incorporating such resins with high functionality conducive to good oxidizing conditions, in particular H2O2. While manufacturers of such polyamide resins make suggestions as to how their products should be employed, these suggestions are merely starting points and do not provide instructions, which can be carried out on a routine basis to formulate a suitable ink or coating formulation. It has been found that the particular combination of components proposed for the present ink or coating formulation of the present invention allows the production of a surface printed or coated film or paper-based packaging material, which may be subjected to the conditions that exist in a typical aseptic food packaging operation. Such conditions are quite severe and normally result in the degradation of the ink or coating on the surface of the package.
The present invention offers ink and coating formulations that may be applied to any film layer or other laminate packaging layer regardless of whether the material is exposed to sterile packaging conditions. Unlike the prior art formulations, there is no need for a protective covering layer for the coated or printed surface of the packaging material.
In one aspect, the present invention provides a non-aqueous coating formulation compatible with aseptic packaging processes which comprises the following components:
1) a mixing varnish comprising a phenolic-modified co-solvent-type polyamide resin,
2) a nitrocellulose compound varnish,
3) a non-aqueous solvent mixture, and
4) specialty additives, and wherein all components present in the formulation are stable in the presence of an oxidizing agent.
In another aspect of the invention, there is provided an ink formulation compatible with aseptic packaging processes which comprises the following components:
1) a mixing varnish comprising a phenolic-modified co-solvent-type polyamide resin,
2) a nitrocellulose compound varnish,
3) a solvent mixture,
4) a non-aqueous solution of a metal salt of propionic acid and
5) specialty additives, and wherein all components present in the formulation are stable in the presence of an oxidizing agent.
In yet another aspect of the invention, there is provided a process for manufacturing a coating formulation that is stable when applied to a packaging material surface and remains so when the film is used to package food products under aseptic packaging conditions, which comprises the steps of dissolving at least one phenolic-modified, co-solvent-type polyamide resin in at least one non-aqueous solvent, and when required combining the above mixture with the following additional components: at least one synthetic wax, at least one slip release agent, at least one nitrocellulose, and one or more specialty additives selected from plasticizers and adhesion promoters.
In another aspect, the invention provides a process for manufacturing an ink formulation that is stable when applied to a packaging material surface and remains so when the material is used to package food products under aseptic packaging conditions, which comprises the steps of dissolving at least one phenolic-modified co-solvent type polyamide resin in at least one non-aqueous solvent, combining the above mixture with a nitrocellulose, at least one pigment and a non-aqueous solution of a metal salt of propionic acid, and when required with at least one synthetic wax, at least one slip release agent, and one or more specialty additives selected from plasticizers and adhesion promoters.
The invention also provides a process for producing a flexible packaging film, a surface of which is printed which film is for use in the manufacture of aseptically packaged food products which comprises the steps of forming a flexible film from at least one resin or one or more film layers made from resin selected from ethylene polymers, polyester polymers, ethylene-vinyl acetate polymers, ethylene vinyl hydroxy polymers and nylon polymers alone or in combination; and applying the ink formulation as defined above to the surface of the film in a desired pattern.
Preferably, the film prior to application of the ink is treated to improve its wettability. The printed film is preferably coated with a clear coating after printing.
In another aspect, there is provided a process for producing a paper-based laminated packaging material a surface of which is printed, which material is for use in the manufacture of aseptically packaged food products, which process comprises the steps of forming a paper-based laminated packaging material and applying to an exterior surface thereof, indicia using the ink formulation as described above.
The printed packaging material may also be coated with a clear coating after printing, which coating is preferably the coating formulation of the present invention.
In yet another aspect, there is provided an aseptic food packaging process comprising the steps of printing the surface of a flexible packaging film with at least one printing ink having the formulation as described above, subjecting the printed, flexible packaging film to a sterilization process for food contact packaging comprising passing the sterilized, printed, flexible film into a sterile zone where a package is formed, filled with a food product, hermetically sealed and cut, and discharging the sealed and packaged food product. The food product may be sterilized.
In another aspect of the invention, there is provided a food packaging process wherein the packaging material comprises a paper-based laminated packaging material that has an exterior printed surface, which process comprises subjecting the printed packaging material to aseptic food packaging conditions wherein a package is constructed from pre-printed paper-based laminated packaging material and the process includes passing the printed material through a sterile zone wherein the food is placed in the package and the package is hermetically sealed.
Preferably, the present invention provides a coating formulation as set out above wherein component 1) is present in the formulation in the amount of from about 40 to about 50% by weight, component 2) is present in the formulation in the amount of about 30 to about 40% by weight, component 3) is present in the formulation in the amount of about 10 to about 30% by weight, and component 4) is present in an amount that comprises the remainder of the formulation up to 100% by weight.
In order to understand the conditions to which the printed film can be subjected during manufacture, distribution and sale, reference is made to the manufacture of sterilized milk pouches using vertical form, fill and seal equipment. However, it will be understood that the present invention may be used in all types of packaging operations where aseptic conditions apply and food material is packaged, regardless of the equipment used.
The vertical form, fill and seal apparatus preferred by the dairy industry is sold under the trade-mark Enhance(trademark) Flexible Aseptic Packaging Systems by DuPont Canada Inc.
Successful aseptic packaging depends on two important operations. Firstly, the packaging material must be sterilized and, secondly, the sterile zone must be maintained while the package is being formed, filled and sealed.
A bath of hot hydrogen peroxide solution is preferred for sterilizing the packaging film. Once the film has been exposed to the bath, it enters a sterile zone where it passes over a forming plate, which folds the material into a tube. The edge of this tube is sealed in a vertica seal unit, and the product is dispensed into the package. The filled package is simultaneously hermetically sealed and cut in a horizontal sealing/cutting unit. The package is then discharged onto a conveyor.
Different sterilization methods may be used in different parts of the apparatus. In the machine itself, a combination of hydrogen peroxide and heated air is preferably used to create the sterile zone, and sterility is maintained by a blanket of incinerated sterile air. Steam is combined with the hydrogen peroxide to sterilize the product supply system.
The size of the package is dependent on the product that is being packaged, but it may vary from small to large, for example, 50 ml up to several litres or more. The advantages of pouch packaging have been well documented in the patent literature, but the advantages may be increased when the pouches themselves contain printed material about the product contained therein, as is possible according to the present invention. Often smaller unprinted pouches are placed in a larger outer bag which contains printed matter about the product. The present invention can eliminate this requirement
The requirements of an ink for applying to the surface of polymer films are very much dependent on the manufacturing conditions and the product being packaged. The printed film must endure any converting operations, such as printing, laminating, coating, and slitting into rolls of correct width. Previously, in order to produce printed packages that would be capable of sterilization in a solution of hot hydrogen peroxide, an extra layer of film was applied via a lamination step or extrusion coating to cover and protect the surface of the printed film.
The sterilization processes that may form part of the packaging operation may be selected from steam autoclaving, ozone gas, ethylene oxide gas, reactive chemicals produced by radiation, hydrogen peroxide bath, and combinations thereof. Preferably, the food product is a flowable material and the package is a flexible film pouch.
The food product package containing a food product may be formed from a packaging material during the packaging of the food product, the surface of the packaging material being printed with the ink formulation as described earlier and remaining stable subsequent to its application to the packaging material, and the material and/or printed film and package is subjected to aseptic packaging conditions and is resistant to abrasion during packing, distribution and sale. Again the food product to be packaged may be sterilized, and the food product may be a flowable material and the package, a flexible film pouch or a paper-based laminated rigid package.
In a preferred form of the present process, the ink coated packaging material may have a clear coat applied thereto at the final station of the printing press. This coating is preferably the coating formulation of the invention. Usually this step is employed in the case of a flexible film packaging material to render the package surface more glossy and to increase scuff resistance. This coating is relatively thinner than coatings used previously.
The polymeric films, which may be used in the present invention, are any of those known in the art of packaging. Any film, which is approved for use with foods, may be used as the flexible film in the present invention. In the packaging of flowable materials, many films are employed and the patent literature is filled with information regarding the nature of these films. Reference in this regard may be had to the disclosures of the following patents and published applications: U.S. Pat. No. 5,747,594 issued May 5, 1998 to de Groot et al.; U.S. Pat. No. 5,792,534 issued Aug. 11, 1998 to de Groot et al.; U.S. Pat. No. 5,879,768 issued Mar. 9, 1999 to Falla et al.; U.S. Pat. No. 5,288,531 issued Feb. 22, 1994 to Falla et al.; U.S. Pat. No. 5,360,648 issued Nov. 1, 1994 to Falla et al.; U.S. Pat. No. 5,364,486 issued Nov. 15, 1994 to Falla et al.; U.S. Pat. No. 5,508,051 issued Apr. 16, 1998 to Falla et al.; U.S. Pat. No. 5,721,025 issued Feb. 24, 1998 to Falla et al.; U.S. Pat. No. 4,521,437 issued Jun., 1985 to Storms; U.S. Pat. No. 5,272,236 issued Dec., 1993 to Lai et al.; U.S. Pat. No. 5,278,272 issued Jan., 1994 to Lai et al.; U.S. Pat. No. 4,503,102 issued Mar., 1985 to Mollison; EP 673397A; and WO 99/10430, the disclosures of which are incorporated herein by reference.
The paper-based packaging material may be printed with the inks of the present invention. The inks may be applied to the polymeric layer that covers the outside surface of the paper layer. A clear coat based on the coating formulation of the invention may be applied, when required, but this is not necessary to protect the printed surface. Reference was made earlier to patents that disclose these types of packaging materials and the packages formed therefrom. Examples are found in U.S. Pat. Nos. 4,424,260, 4,416,667 and 4,495,016, the disclosures of which are incorporated herein by reference.
A particularly preferred co-solvent-type polyamide resin is UNI-REZ(copyright) 1533, manufactured by Arizona Chemical Company.
When pigments are employed in the formulation of the present invention, a solution of the metal salt of propionic acid, preferably zirconium propionate, is used as an adhesion promoter for the pigment and has been found to be essential to the successful application of the ink to the film when the film is printed under aseptic packaging conditions.
The typical ranges for the components of the ink formulation of the present invention are as follows:
from 10 to about 55 percent by wt. of a nitrocellulose base dispersion;
from about 45 to about 55 percent by wt. of a mixing varnish comprising a non-aqueous solution of a polyamide resin; and
from 1 to about 2 percent by wt. of a non-aqueous solution of zirconium propionate additive.
Examples of suitable formulations that lie within these ranges are found subsequently.
The nitrocellulose compound varnish preferably comprises xc2xc sec SS nitrocellulose and preferably a mixture of solvents, for example, at least one alcohol and at least one alkyl acetate, together with a fatty amide. Nitrocellulose release varnish is present in the amount of about 15% by weight, and normally this varnish comprises about 30% by weight of compound varnish solution, about 30% by weight ethanol, about 15% by weight n-propyl acetate and about 25% by weight of a fatty amide, preferably a hydrogenated tallowamide. A commercial example of such a tallowamide is ARMID HT(copyright) supplied by Akzo Chemicals Inc.
In a preferred formulation of the present invention, the mixing varnish solvent system comprises about 32% by weight of propyl alcohol, about 9.8% by weight aliphatic hydrocarbon, about 9.0% by weight n-propyl acetate, about 4.0% by weight of a synthetic wax which is typically a free flowing powder that aids anti-block, lubricity and abrasion resistance, and from about 15% to about 20% by weight of release varnish. A commercial example of such a product is Shamrock Synthetic Wax, which is a micronized polypropylene wax. In a preferred form, about 0.2% by weight of Dow Corning Silicone Fluid is added for slip and release of the heat sealing jaws typically used in the preferred vertical form, fill and seal packaging process of the present invention.
Preferably, in the ink formulation, the phenolic-modified, co-solvent-type polyamide resin is present in an amount ranging from about 20 to about 35% by weight or more preferably about 30% by weight of the resin. Preferably, the mixing varnish comprises about 45 to about 50% by weight of the overall ink formulation, with the colour concentrate comprising about 25 to about 50% by weight.
A preferred amount of the non-aqueous solution of zirconium propionate may comprise about 1.5% by weight of the formulation. A typical example of such a composition is a 50:50 composition of zirconium propionate and ethanol. Suitable amounts of other metal salts may be selected based on this amount.
The pigment used in the ink formulation of the present invention may be any commercially available pigment approved for use in connection with food, in particular on the outside of a food package. It is most important that the pigment be resistant to hydrogen peroxide, or any other sterilization means, which may be used in packaging food with the printed packaging materials of the present invention.
Typical pigments, which may be used in the formulation of the present invention, are as follows
Typically the above pigments will be formulated into a colour concentrate by adding them to a solution of xc2xc sec nitrocellulose. These dispersions are then blended in various combinations and percentages to a combined total of no higher than about 55% of the total ink formulation or where necessary to the optimum value required to achieve the desired colour intensity.
Preferably the specialty additives may comprise from about 1 to about 3% by weight of the formulation, when present. The plasticizers promote adhesion in the ink formulation. If present in excess, they may cause blocking. Examples of the specialty additives include polyethylene wax and silicone fluid. The sterilization bath comprising hydrogen peroxide employs food grade hydrogen peroxide at a temperature of 50xc2x0 C. and a concentration of between 30-35 weight percent. Both sides of the film are fully immersed in the peroxide for a period at least about 30 seconds in the process of the present invention.